A 28-year-old man with a known history of fasting-induced seizures since childhood is admitted with recurrent hypoglycemia (blood glucose 35–50 mg/dL) despite a 6-hour fast. His parents report that he requires frequent meals and cannot tolerate prolonged fasting. Liver ultrasound shows hepatomegaly with a smooth, non-cirrhotic appearance. Serum lactate is 8 mmol/L (normal < 2), and uric acid is markedly elevated at 9.2 mg/dL. Which of the following metabolic derangements is the PRIMARY consequence of the underlying enzyme defect?